Susceptibility of Cider Apple Accessions to European Canker—Comparison between Evaluations in Field Planted Trees and Rapid Screening Tests

Abstract

European canker, caused by Neonectria ditissima Bres., is an economically damaging fungal disease of apple. Breeding new cultivars with a high level of resistance to European canker is the main aim of apple breeding programs. Observations of symptoms in naturally infected trees were carried out in 400 apple accessions in Asturias (north-western Spain). Young and mature field planted trees were assessed under conditions highly conducive for N. ditissima development. The results demonstrated that juvenile trees (4-year-old) barely showed noticeable symptoms whereas a wide variability in the levels of resistance among accession was observed in mature trees (14-year-old). Around 28% of the locally maintained collection resulted to be highly resistant to this disease in the region. Field observations on mature trees were also compared to four rapid screening tests based on artificially induced lesions. Spearman correlation analysis using two resistance parameters revealed that none of the methods resulted in similar rankings of cultivar susceptibility as some accessions that were ranked as resistant for a given test turned out to be susceptible in the field. This study might suggest that whilst conventional resistance phenotyping techniques are time-consuming, the outcomes of this approach still seem the preferred option to assess the response to N. ditissima of apple accessions.

Apple genomics for the Swedish breeding programme

“…While it has been questioned whether screening tests in controlled conditions are relevant to field conditions (Delgado et al 2022), there are large differences between experimental set-ups that might appear similar at a first glance, e.g., potted trees experiment. also made use of a controlled conditions experiment with potted trees, which in some respects was similar to the experimental set-up of the current study.…”

“…Despite the importance of the disease, there is currently no consensus on how to screen for resistance. Reported approaches differed in type of plant material (trees, detached shoots), cultivation (planted, potted, outdoors, or in a glasshouse), sources of inoculum (e.g., wild collected vs single-spore isolate), type of isolates (wild collected and in vitro propagated single-spore isolates), and type of spores (ascospores, conidia) as well as wounding techniques (e.g., naturally occurring leaf scars or manually inflicted by rasp or scalpel) (Bus et al 2019;Delgado et al 2022;Gómez-Cortecero et al 2016;van de Weg 1989;Wenneker et al 2017). Also, various parameters to quantify resistance have been used, including area under the disease progression curve (AUDPC), infection rate, the slope of the lesion growth regression line, lesion length at a given timepoint, and indexes quantifying the establishment of secondary infections (Bus et al 2019;Gómez-Cortecero et al 2016;Wenneker et al 2017).…”